Premium intraocular lenses commonly implanted during cataract surgery are categorized in three ways: accommodating, multifocal and toric intraocular lenses
The best visual acuity is achieved with the single focus accommodating lenses. The optic of these lenses moves forward and backward upon constriction and relaxation of the ciliary muscle. However, for reading in dim lighting conditions, or for small print, weak reading glasses are often necessary.
Multifocal lenses focus light on the retina at either two or three focal lengths. Thus, there is more than one image on the retina simultaneously forcing patients to select the image they wish to perceive, which poses a problem for patients unable to adjust their focal point. This creates additional problems in that the amount of light in focus is divided between the multiple focal points, and contrast sensitivity is thereby reduced, making vision at all distances difficult in dim lighting. In addition, there are severe problems when driving at night when the pupil is dilated. Many patients experience severe glare and halos and many have had to have the multifocal lenses explanted and replaced with a single vision standard lens, because of this problem. However, the near vision with the multifocal lenses is superior to that of the current accommodating lens.
Toric lenses correct eyes that have significant astigmatism.
The currently marketed plate accommodating intraocular lenses provide excellent distance and intermediate vision but sometimes require weak, +1.00, reading glasses for prolonged reading, for seeing small print, or reading in dim lighting conditions.
It is desirable to provide a single vision intraocular lens that will allow seamless vision at all distances. However, without excellent uncorrected distance vision there is no point in implanting lenses designed to give seamless vision from far to near.
Furthermore, it is important for intraocular lenses to have a consistent location along the axis of the eye to provide good uncorrected distance vision and to center in the middle of the vertical meridian of the eye.
The original intraocular lens consisted of a single optic. These lenses frequently de-centered and dislocated and it was discovered that there was a need to center and fixate the lens optic in the vertical meridian of the eye.
Attachments to the optic that center and fixate the lens within the capsular bag are called haptics. Traditionally, haptics consist of multiple flexible loops of various designs, J loops, C loops, closed loops and flexible radial arms. Recently, traditional haptics have been replaced in some lens designs with oblong, flat flexible plates, called plate haptics. These plate haptics usually made from silicone, are solid, flat, flexible and between 3.0 and 6.0 mm in width, 0.20 to 0.75 mm thick, and may have tapered, rounded or parallel sides. Plate haptics often have flexible loops or fingers that help center and fixate the lens within the capsular bag. These flexible fingers extend beyond the distal or outer end of the plate haptics and slightly beyond the diameter of the capsular bag and are designed to flex centrally to center and fixate the lens and its optic within the capsular bag.
An intraocular lens (IOL) is a lens implanted into the eye, usually replacing a normal human lens that has been clouded over by a cataract, or can replace a normal human lens as a form of refractive surgery to change the eye's optical power.
An accommodating IOL (AIOL) permits refocusing by means of movement along the optical axis in response to the constriction or relaxation of ciliary muscles. Near vision results from a forward movement of the optic on constriction of the ciliary muscle, which causes an increase in the pressure in the posterior part of the eye with a simultaneous decrease in pressure in the anterior part of the eye. Distance vision results from the reverse pressure change that takes place upon relaxation of the ciliary muscle and the resultant backwards movement of the lens. The movement of the optic enables the patient implanted with the lens to automatically change their vision between far, intermediate and near.
IDLs are known to consist of opposing haptics positioned on either side of a lens optic. Once a patient's cataract is removed, by e.g. phacoemulsification, the IOL is folded and placed into the capsular bag via what is typically a 3-4 mm incision. The haptics help to center the IOL and fixate it within the capsular bag by fibrosis. Such AIOLs are described in U.S. Pat. No. 5,674,282, U.S. Pat. No. 5,476,514, and U.S. Pat. No. 5,496,366, to Cumming, herein incorporated by reference in its entirety.
However, due to its construction, the lens optic of a traditional plate haptic lens is limited in its response to the change in vitreous pressure. Furthermore, when a plate haptic lens is placed within the capsular bag of the eye the peripheral circumferential remains of the anterior capsule and the posterior capsule of the human capsular bag, fibrose over the distal ends of the plates. The area of fibrosis can vary and sometimes covers only the distal 1.0 mm of the tip of the plate. With inadequate coverage of the distal ends of the plates the plate haptics can sometimes dislocate, one of the plates vaulting forwards to configure the lens in a “Z” shape configuration.
Moreover, these designs do not permit adequate movement of the optic to a change in vitreous cavity pressure to allow many patients to read comfortably at near without glasses. In order to increase the movement of the optic to respond to the increase in vitreous cavity pressure that occurs during ciliary muscle constriction, the transverse hinge connecting the haptic and the optic may be weakened by elongating the hinge or reducing its width. However, such alterations tend to destabilize the lens optic and make it prone to tilting.